Rather than using TIPxxx series darlington transistors, you would be better
off using discrete single transistors to build your darlington, so that the
power transistors get better saturation voltages. Your TIPxxx will have a
saturation voltage o about 0.9V, while using discrete single transistors to
build darlingtons should achieve saturation voltages around 0.25-0.3V quite
easily.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.